As well known, control rods in a nuclear reactor perform the functions of power distribution, shaping and reactivity control. This is accomplished generally by providing a plurality of control rods containing material for neutron absorption and manipulating the control rods within the reactor. Generally, a control rod used in a boiling water reactor (BWR) is provided in a cruciform shape and extends in complementary cruciform-shaped interstices between fuel channels. While there are a number of different control rod designs, two basic control rod designs have been typically employed previously in boiling water reactors. In one design, each control rod consists of a plurality, e.g., either 48 or 84, boron carbide absorber or hafnium rods, or a combination of boron carbide and hafnium rods, separated into four cruciform-shaped wings of either 12 or 21 tubes each. The tubes are enclosed in a perforated outer sheath which is welded to a full-length central tie rod which has a handle and connector/velocity limiter at the respective opposite ends of the rod. The perforated sheath affords a cooling medium to the tubes. If a tube fails in this design, the boron carbide powder is exposed to the reactor coolant.
In another type of control rod, essentially square tubes are welded to one another to form the four wings of the cruciform-shaped control rods. Each wing contains 12 to 15 square tubes welded to one another and to a handle at one end and a connector/velocity limiter at the opposite end. The welds between the tubes and the tubes themselves provide the structural support for the control rod. The tubes serve as individual pressure vessels, as well as structural members subjected to all reactor induced loadings. In this design, the boron carbide powder is contained in sealed capsules inserted into the individual square tubes. One of the difficulties with the latter type of control rod construction is the magnitude of the welding required to weld each of the individual square tubes to an adjacent tube to provide the structural support necessary for the four wings of the control rod. While that construction has served well, it is quite expensive to manufacture.
Another problem associated with the design of control rods is that the reaction of the neutron-absorbing material, e.g., boron carbide, with neutrons produces a helium gas. Thus, the tubes of each of the previously noted designs must have structural integrity to maintain the generated helium gas within the tube at increasing pressure throughout the lifetime of the control rod. It is known that the mechanical lifetime of a control rod is limited by the reactor burn-up and the corresponding helium pressure build-up in the tube containing the boron carbide exposed to the highest neutron flux. The magnitude and pressure of helium gas generated depends on the location of the tube within the control rod. That is, the boron carbide containing tube which has the highest exposure to neutrons in the control rod determines the mechanical life of the control rod. However, in each prior design, the neutron-absorbing material was contained in discrete tubes individually subjected to helium gas pressure build-up and without any relief. Consequently, the life of the control rod is dependent upon the structural integrity of only one of a large number of absorber tubes.